The present invention relates in general to a cell for the electrolytic production of metals by electrolysis of anhydrous metal halides in a bath of molten salts, and in particular, to the electrolytic production of aluminum from the corresponding anhydrous chloride.
Experts in the art have long referred to the process of igneous electrolysis of alumina in a molten mixture of sodium and aluminum fluorides in the design of arrangements for igneous electrolysis of anhydrous aluminum chloride in a bath of molten salts.
The large number of literature references in this field is the consequence of certain observations made by experts, concerning the advantages that such a process might have over the Hall-Heroult process; e.g., that electrolysis could take place at a lower temperature, and that there could be a reduction in the consumption of electrodes through oxidation of their constituent graphite, due to the oxygen which is liberated during electrolysis of the alumina.
Several major disadvantages, however, soon became apparent and prevented the process from being exploited commercially.
Indeed, as expressed in French Pat. No. 2,158,238, experts were confronted with particularly troublesome phenomena, since the most serious disadvantages are due to the presence of dissolved or non-dissolved metal oxides, such as alumina, silica, titanium oxide or iron oxide in the electrolytic bath.
The non-dissolved metal oxides result in a gradual accumulation of a viscous layer on the graphite cathodes. The viscous layer comprises finely divided solids, liquid constituents of the bath and droplets of molten aluminum. These components impede access to the cathodes of the electrolytic bath and may cause problems in the normal cathode mechanism. Specifically, the components may lead to reduction of the cations containing the metal which are produced at various stages of oxidation. Thus, the aluminum chloride which is present in the viscous layer then consumed by electrolysis becomes increasingly difficult to replenish, and consequently the other chlorides in the bath of molten salts may be electrolyzed, resulting in a loss of efficiency in the electrical energy used and pollution of the metal.
Moreover, because the chlorides forming the bath of molten salts including alkali chlorides (e.g., sodium, potassium and/or lithium) and alkaline earth chlorides (e.g., magnesium, calcium and/or barium), are partially electrolyzed, incomplete renewal of the aluminum chloride near the cathode takes place producing the corresponding metals, which are inserted by cathode potential in the graphite of the electrodes and cause the cathodes to disintegrate and crumble. This premature wear on the cathodes causes particles of carbon to enter the bath. Those particles contribute to the formation of sludges at the cathode and lead to a reduction in yield.
Finally, another equally serious disadvantage, which has to do with the presence of dissolved metal oxides in the bath, such as alumina, is that oxygen is liberated at the anode and consumes the carbon which forms the anode. This consumption upsets the electrolytic operation, since it changes the geometric properties of the anode and particularly the distance between the anode and cathode.
Since the above-mentioned disadvantages were caused by the available processes, experts directed their research efforts to an apparatus for igneous electrolysis of anhydrous aluminum chloride in a bath of molten salts.
Apart from the above-mentioned drawbacks, what had to be studied and resolved was how to obtain a high yield of electrical energy, for example, by means of a low voltage and a high yield of current, while limiting any inverse reaction of the chlorine-aluminum type.
Thus, experts proposed the use of cells with bipolar electrodes to obtain at least some of the desired improvements mentioned above. Cells of this type have been produced, enabling such electrodes to be used either in a horizontal or inclined position so that the metal formed on each cathode surface is deposited in the bottom of the cell by gravity, and so that the chlorine produced on each anode surface is displaced in the opposite direction to the metal. Therefore, the chlorine migrates freely to the top of the cell without establishing any contact with the liquid metal.
A cell of the above type with bipolar electrodes is described in French Pat. No. 2,152,814. The cell comprises, horizontally stacked and in descending order, an anode, at least one intermediate bipolar electrode and a cathode, which are superposed and evenly spaced by insulating refractory struts. Substantially horizontal spaces are thereby created between the electrodes, for the purpose of electrolyzing the aluminum chloride in a bath of molten metals in each interpolar space. This leads to the liberation of chlorine from each anode surface and the deposit of aluminum on each cathode surface.
As a means for making the bath circulate well into each interpolar space and encouraging the metal formed to be carried out of the spaces, the chlorine produced performs the function of a delivery pump, carrying the lightest part of the bath to the surface and encouraging the aluminum obtained to be decanted to the bottom of the cell, both through appropriate passages. To this end, each bipolar electrode is equipped with an absolutely flat cathode surface and an anode surface with transverse hollowed channels.
Thus, each anode surface comprises a plurality of such channels extending transversely to the lateral edge of each electrode at the side where a passage is reserved for the return of the bath and the ascent of the gas. The purpose of the channels is to keep the chlorine that is liberated from the interpolar space away from the aluminum deposited on the cathode surface to limit rechloridation of the metal produced.
Another cell of the above type, with bipolar electrodes, is described in French Pat. No. 2,301,443, and is an improvement of the cell described in French Pat. No. 2,152,814. The cell comprises, in descending order and placed horizontally, an upper anode; intermediate bipolar electrodes, which are stacked on one another with spaces therebetween and maintained at equal distances by insulating refractory struts creating regular, virtually horizontal inter-electrode spaces, each space being defined at the top by the bottom surface of an electrode which acts as an anode surface, and at the bottom by the top surface of an electrode which acts as a cathode surface; and a bottom cathode.
As in the previously described patent, the anode surface may contain transverse channels to encourage the chlorine to flow out of the inter-electrode space to a zone for the ascent of gases. The channels are formed in the central part of the cell between the stacks of electrodes, the zone widening out from the bottom towards the top of the cell. Thus, the provision of channels on the anode surface leading to a zone for the ascent of gases, formed in the central part of the cell between the stacks of electrodes, is designed to rapidly remove the chlorine liberated from the interpolar space, but above all to remove the aluminum deposited on the cathode surface in order to limit its rechloridation.
Although such technology can bring substantial, remarkable improvements in the electrolysis of aluminum chloride, it has to be admitted that the arrangements proposed still have serious enough disadvantages to hamper their optimum use in industrial processes.
Apart from the fact that channels for discharging the gases liberated have to be provided on the anode surface to prevent the gases from accumulating in the interpolar space, thus making the industrial production of this type of electrode particularly expensive, such electrolytic cells are the subject of many disturbances relating to the existence of parasitic tapped currents produced by non-consecutive electrodes positioned too close together.
Moreover, such electrolytic cells suffer from thermal imbalance as a result of the disproportion between the energy dissipated at the center of the cells and the external radiating surface.
Finally, such cells suffer from the continued existence of the droplets of aluminum produced only a short distance away from the anode. This creates a considerable danger that a fraction of the aluminum produced may be reoxidized; such reoxidation would upset the thermal balance because of its exothermic character.
With a realization of the interest which experts in the field would have in a new cell, well adapted to the electrolysis of metal halides, and more particularly aluminum chloride in a bath of molten salts, but also with an awareness of the disadvantages attached to the techniques previously described in this field, applicants have continued their research and have designed and perfected an improved cell for the electrolysis of these halides, which is virtually free from the described disadvantages.